1. Field of Invention
The invention relates to a shift control apparatus for a vehicular automatic transmission and a shift control method, which allow a vehicle operator to perform manual selection of gear stages.
2. Description of Related Art
When the gear stage is in a drive (D) position operated by the vehicle operator using the shift device, a shift control apparatus of the automatic transmission for the vehicle serves to select an appropriate gear stage in accordance with the current operating state of the vehicle. Then a hydraulic control portion of the automatic transmission is controlled such that the gear is automatically changed to the selected stage.
The operating state of the vehicle is determined by the shift control apparatus based on a throttle opening and a vehicle speed, for example, and the gear stage is selected so as to provide the fuel efficiency and power most suitable for the operating state.
There has been introduced an automatic transmission that allows the vehicle operator to manually change gear stages in order to realize not only the aforementioned function but also the vehicle operation that reflects the intention of the vehicle operator.
In the shift control apparatus in the automatic transmission for a vehicle disclosed in JP-A-6-221417, for example, the shift device includes ranges D, and B that can be selected from the range D by manual operation of the vehicle operator. The shift device further includes an upshifting position and a downshifting position that can be selected in the range B. An electronic control unit that serves to perform shift control of the automatic transmission becomes an automatic shifting mode in the range D such that the appropriate gear stage can be automatically selected in accordance with the operating state of the vehicle. Upon change in the shift range from the range D to B, the electronic control unit sets a manual transmission mode where the gear stage is downshifted so as to be lower than the gear stage that has been selected in the range D. When the vehicle operator performs downshifting in the range B, the electronic control unit downshifts the gear to be one stage lower. Meanwhile, when the vehicle operator performs upshifting in the range B, the electronic control unit upshifts the gear to be one stage higher.
In order to obtain the engine braking force higher than that obtained in the gear stage that has been set during running at the automatic transmission mode, the vehicle operator changes the shift range from the range D to B using the shift lever so as to perform downshifting to the gear as being one stage lower. In order to obtain higher acceleration torque or the torque for running uphill, the aforementioned change in the shift range is performed to establish the gear to be one stage lower.
In the state where the automatic transmission includes 5 shift speeds, when the vehicle operator releases the accelerator pedal during running in the 3rd speed at the automatic transmission mode, upshifting from the 3rd to the 5th speed is performed by the electronic control unit in response to the operating state of the vehicle, that is, the state of the throttle valve which has been fully closed.
When the vehicle operator changes the shift range from the range D to B for downshifting to apply further higher engine braking force, the downshifting is performed from the 5th to the 4th speed. Then further downshifting in the range B changes the gear stage from the 4th to the 3rd speed.
In the above case, the engine braking force required by the vehicle operator cannot be obtained because the gear stage has been originally set in the 3rd speed at the automatic transmission mode. The vehicle operator, therefore, has to perform further downshifting from the 3rd to the 2nd speed.
Depending on the operating state of the vehicle, the vehicle operator has to perform downshifting twice so as to obtain the required engine braking force. Even in the state where the 5th speed is set in the range B at the manual transmission mode and the vehicle operator releases the accelerator pedal, the gear is held in the 5th speed by the electronic control unit.
If the vehicle is running downhill in the aforementioned state, the vehicle will be accelerated without depressing the accelerator pedal. When the vehicle operator performs downshifting once for the purpose of applying the higher engine braking force just before entering to a sharp curve while running downhill, the gear stage is shifted from the 5th to the 4th speed.
As the engine braking force obtained in the 4th speed is insufficient in the above case, the vehicle operator has to perform further downshifting once to the 3rd speed.